Conventionally, an erasing apparatus which erases a decoloring image printed on a sheet through heating with a print image erasing unit to re-use the sheet is suggested. The print image erasing unit heats the sheet over its entire length in the width and length directions.
In other words, the print image erasing unit consumes heating energy to erase the image over the entire surface of the sheet.
For this reason, even if characters are printed over the entire surface of a sheet, an erasing process is performed with the same heating energy regardless of the value a print ratio that represents the proportion of characters occupying the whole sheet.
In addition, even when images are scattered in a portion of a sheet, other portions with no images are heated.
Thus, a technology wherein the printed surface of a sheet before an erasing process is read by a scanner and the state of the printed surface is detected based on the read image is suggested.
However, a method in which the print state of a sheet is optically detected as with a scanner or the like requires a high cost, and it is difficult to reduce the size of the erasing apparatus of the method in that space to install the optical system is needed.